Online Medical Reference

Nutrition

Roxanne B. Sukol

Published: November 2013

Definition

"Nutrition," derived from the Latin nutrire and related to both "nourish" and "nurse," means to feed, support, or nurture. "Food," from the Old English foda, means nourishment or fuel, and is related to the words fodder and feed. The purpose of food, then, is to nourish.

Whereas there is significant controversy about what constitutes good nutrition, there can be none about the skyrocketing rates of obesity and diabetes. There is general agreement that the cost, in both human and monetary terms, is unacceptable and unsustainable; and the urgency of the problem cannot be overstated. How did we reach this point, and what can be done about it? Insight into normal physiological processes is often advanced by parsing the mechanisms of disease caused by even minute variations to those processes. In a similar way, this section will examine how several major changes in our food supply may have resulted in the epidemic of obesity, diabetes, and related chronic illness in which we currently find ourselves.

Nutrition and weight loss advice abound, but we have been unable, thus far, to identify a consistent format for providing that advice in a way that makes sense to patients and helps them to successfully navigate the maze of food-related decisions that they encounter daily. This section will examine technology-driven changes to the food supply that occurred through the 20th century, and then apply this information to explain why recommendations to increase the purchase and consumption of carbohydrates with an intact fiber matrix (vegetables, legumes, fruit, whole grains) while decreasing the purchase and consumption of processed items (chips, coffee whiteners, carbonated beverages, commercially baked goods, fast food) constitute a reasonable and reliable initial strategy for normalizing body weight.

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The Industrialization of Food

The patterns of food supply, distribution, preparation, and consumption have undergone a marked change in the past 100 years. Prior to the 20th century, for example, most Americans ate meals that they prepared themselves in their own homes. Today, in contrast, not only are the majority of meals eaten outside the home, but meals eaten at home are often made elsewhere as well. For the first time in history, vast amounts of edible items are now being raised, prepared, and/or manufactured by entities unknown to the end consumer. Previously, meals consisted almost entirely of some combination of fruits, vegetables, legumes, nuts, fish, eggs, meats, poultry, whole grains, and dairy products, such as milk, cream, butter, yogurt or cheese. Manufactured items such as corn syrup and partially hydrogenated vegetable shortening, for example, did not exist in the food supply.

In addition, the meanings of food-related words have changed dramatically. Whereas the word "wheat" once referred to a grain in its entirety–including the bran, endosperm, and germ–in current usage it means the endosperm alone. The original product is distinguished as "whole-grain wheat". As the former meanings of many food words have been appropriated to describe processed food products, traditional staples have come to require new descriptors. This explains the widespread need for terms like "organic," "pesticide-free," "wild," "free-range," "whole," "old-fashioned," "pastured," and "hormone-free".

In supermarkets, processed items are displayed adjacent to the products they are intended to replace. To increase the likelihood of purchase, sealed containers of non-dairy creamer, which require no refrigeration, are located in the cold section with milk and cream. The first two ingredients in almost all non-dairy creamers, whether powder or liquid, are high-fructose corn syrup and partially-hydrogenated vegetable oil. Because low production and material costs tend to make processed products highly profitable in comparison to traditional foodstuffs, large amounts of supermarket shelf space are dedicated to them.

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Food Chemistry

The explosion in the number of processed edible items developed throughout the 20th century became possible largely as a result of two innovative approaches to managing fat oxidation, the major cause of food rancidity. The first approach consisted of removing the oil-rich germ (as well as the brown, fiber-rich bran) from whole grains to make "white" flour. The second approach involved a transition to the use of fats with a lower oxidation potential. This was achieved through either 1) partial hydrogenation, which converted oil from a liquid to a solid state; or 2) selective and extensive use of omega-6 polyunsaturated fatty acids. These two developments increased shelf life markedly, and this, in turn, was the key to building the food industry.

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Commerce and Technology: Macronutrients

Besides water, macronutrients in food consist of carbohydrate, fat, and protein. Changes to macronutrients first affected carbohydrates and fat. Sources of animal protein were affected later, and more indirectly, as animal populations (i.e., primarily mammals and poultry) raised for human consumption were transitioned to grain-based diets, and as other standard management practices changed in response to evolving economic demands.

Carbohydrate

A visit to any garden, orchard, or meadow will demonstrate that carbohydrate-rich foods grow within a fiber matrix. Until recently in the course of human evolution, that is how almost all carbohydrates were consumed. Only a small amount of fiber-less carbohydrate would have been available in the food supply, primarily as dairy products and honey.

In contrast, most of the carbohydrate available today has been stripped from its fiber matrix. "Stripped carbohydrate" originates either as sugar cane, dates, or beets; or as grain (e.g., wheat, rice, corn). Extraction of table sugar (i.e., sucrose) from cane appears to have originated in India and China. The technology is thought to have reached the Mediterranean basin somewhere between 800 and 1100 AD. By the 14th and 15th centuries, sugar was being exported, in small quantities, for "medicinal purposes," from Italy to England. Demand began to rise after the introduction of tea and coffee.

Before the late 1700's, bread was a chewy, dense, brown loaf. By the late 18th century, however, profitable experiments like George Washington's state-of-the-art grist mill in Mt. Vernon were yielding a finely ground, white flour that was shipped to Great Britain and other parts of Europe for purchase by members of the upper class. "Refined" flour was costly and available only to individuals of means.

Advances in technology and commerce resulted in the development of major global channels for the manufacture and distribution of enormous quantities of sugar and white flour. Manufacturing costs were further decreased by economies of scale and government subsidization of raw materials. Consumption of stripped carbohydrates rose steadily until, by the end of the 20th century, they had come to constitute a significant majority of the items available for purchase in most supermarkets and many restaurants. In the second half of the 20th century, another form of stripped carbohydrate was introduced to the market in the form of high-fructose corn syrup. High-fructose corn syrup was invented in 1957 and incorporated into the mass market in the mid-1970s. Because of its low cost relative to sugar, as well as its liquid state, which facilitated ease of mixing, it was immediately adopted as a substitute for sugar throughout the industry, especially for sweetening commercially baked goods, condiments, and carbonated beverages.

A major source of stripped carbohydrate in the American diet is beverages. American children consume an average of 500 cans of soda annually, each containing 10 teaspoons (40 grams) of sugar, usually as less expensive high-fructose corn syrup. Fruit drinks, marketed heavily as an alternative, especially for consumption by children, also contain large amounts of sugar. It is notable that although stripped carbohydrate became broadly available a comparatively short time ago, standard American breakfast recommendations – toast, bagels, muffins, waffles, pancakes, biscuits, breakfast cereals and bars – appear to consist almost entirely of sugar and stripped grains.

Excessive consumption of stripped carbohydrates spikes blood sugars, increases endogenous insulin demand, facilitates fat deposition, and increases the incidence of chronic disease. Among the myriad of recent attempts to reverse these rising trends, a "glycemic index" was invented to compare rates of absorption and resulting blood sugar excursion following carbohydrate ingestion. Slowly absorbed items have lower ratings and tend to be whole, or intact, carbohydrates; quickly absorbed items have higher ratings and tend to contain significant amounts of stripped carbohydrate. Watermelon and dates are two of the few intact carbohydrates that cause rapid blood sugar excursions. Limiting intake of stripped carbohydrate would, therefore, obviate the need for a sugar absorption index.

Fat

Our understanding of the role of fats in our diets continues to evolve. The Mediterranean diet lowers the risk of cardiovascular disease, yet contains many nutritious fats such as fish, nuts, seeds, avocados, olives, and eggs. This observation, surprising at first, makes a good deal of sense if we turn our focus from fats as a general category to fats specifically selected and/or modified to delay oxidation.

Food science advances focus heavily on fat oxidation, the major cause of food rancidity and instability. Over the course of the 20th century, two technological approaches were identified to decrease chemical susceptibility to oxidation, and to increase shelf life. These were 1) partial hydrogenation and 2) increased usage of omega-6 polyunsaturated fatty acids.

Trans fats

Partially-hydrogenated vegetable oil, or "trans" fat, was first synthesized from cottonseed oil for the soap and candle-making industries as an inexpensive alternative to tallow and lard. Not long afterward, the commercial baked goods industry was identified as a lucrative third market. The chemical stability of partially-hydrogenated cottonseed oil conferred a much longer shelf life on food items made with it. Unfortunately, however, while it behaved like other fats in the preparation of food, its considerable role in the pathogenesis of cardiovascular disease remained silent and misunderstood for decades.

Omega-6 and Omega-3 Fatty Acids

Omega-6s and omega-3s are essential fatty acids. Prior to the 20th century, virtually all human diets contained balanced amounts of omega-3 and omega-6 fatty acids, in a ratio very close to 1:1. As processed items came to comprise a growing proportion of the current American diet, the ratio climbed to an estimated 15:1 to 20:1 or more. Because it is the ratio of these two fatty acids, one to the other, that determines their nutritional value, they will be discussed in combination.

Humans cannot synthesize omega-3 and omega-6 fatty acids, and that is what makes their presence in the human diet essential. However, these two polyunsaturated fatty acid subtypes serve markedly different and complementary purposes, both in the plants from which they originate near the bottom of the food chain, and in the animals that consume them.

Omega-3 fatty acids are found in green vegetables, especially algae and leafy vegetables. They exist in high concentrations in chloroplasts, where they play a central role in photosynthesis. Omega-3 molecules are often highly flexible, which makes them ideal for providing structure to green vegetables, especially leaves. Their major limitation, however, is that they are readily oxidized, and this makes them unreliable for fat storage.

With fewer double bonds and greater molecular stability, omega-6s are the preferred form of fat in seeds and grains, where fat is stored until germination occurs and photosynthesis begins. In other words, those molecular properties that make omega-6 fatty acids stable and oxidation-resistant are precisely those which make them ideal for use in processed food.

Upon germination, plants release an enzyme, omega-3-desaturase, to convert stored omega-6 fatty acids into omega-3s. Most mammalian cells, with the exception of certain obligate carnivores, lack omega-3-desaturase and are therefore unable to convert ingested omega-6's to omega-3's. Also, mammals generally convert omega-6 fatty acids primarily to arachidonic acid, a precursor to pro-inflammatory cytokines. Conversely, omega-3s are an important molecule in the cascade of anti-inflammatory cytokines, which includes EPA (eicosapenta–enoic acid) and DHA (docosahexaenoic acid) (Table 1).

Table 1. Cytokine Synthesis via Omega-3 and Omega-6 Pathways
1. Alpha-Linolenic Acid (ALA) → Omega-3 Fatty Acids
Alpha-linolenic acid (ALA) →(delta-6-desaturase)→ Stearidonic Acid →(elongase)→ Eicosatetraenoic Acid →(delta-5-desaturase) → eicosapentaenoic acid (EPA):
#1 EPA → (cyclooxygenase) → anti-inflammatory Series 3 prostaglandins.
#2 EPA → (5-lipoxygenase) → pro-inflammatory leukotrienes.
#3 EPA → (elongase) → docosapentaenoic acid → (delta-4-dehydrogenase) → docosahexaenoic acid (DHA).
2. Linoleic Acid (LA) → Omega-6 Fatty Acids
Linoleic Acid (LA) → (delta-6-desaturase) → Gamma-Linolenic Acid (GLA) → (elongase) → di-homo-GLA → (delta-5-desaturase) → anti-inflammatory Series 1 prostaglandins or pro-inflammatory arachidonic acid (AA):
#1 AA → (COX [cyclooxygenase]-2) → pro-inflammatory Series 2 prostaglandins.
#2 AA → (5-lipooxygenase) → pro-inflammatory leukotrienes.

Data from Simopoulos AP. The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases [published online ahead of print April 11, 2008]. Exp Biol Med (Maywood) 2008; 233:674–688. doi:10.3181/0711-MR-311

Following the desaturation of omega-6s to omega-3s by omega-3-desaturase, a second enzyme, delta-5-desaturase, plays a significant role in both the omega-3 (derived from alpha-linolenic acid) and omega-6 (derived from linoleic acid) pathways. Normally, delta-5-desaturase has a greater affinity for omega-3 than omega-6. In the setting of an omega-3 shortage, however, it will preferentially generate pro-inflammatory arachidonic acid (AA). Therefore, a diet high in omega-6s and low in omega-3s will shunt large amounts of omega-6 linoleic acid (LA) into pro-inflammatory arachidonic acid (AA), which will leave less delta-5-desaturase to convert alpha-linolenic acid (ALA) to the anti-inflammatory omega-3 eicosapentaeonic acid (EPA).

Another notable omega-3 fatty acid is docosahexaenoic acid (DHA). DHA is a long, highly flexible molecule, with hundreds of possible configurations. This flexibility makes it ideally suited as a component of cell membranes, whose primary function is to separate internal and external environments while concurrently facilitating controlled communication. DHA constitutes approximately 25 percent of brain tissue, has anti-arrhythmogenic properties, increases levels of light-responsive rhodopsin in the retina, and may lower the risk of obesity and diabetes by enhancing insulin sensitivity. Insulin resistance, elevated blood pressure, inflammation, and platelet aggregation are consequences of a diet high in omega-6s and low in omega-3s. Omega-3s have been associated with beneficial effects to bone mineral density in males.

Whereas warm-blooded mammals require 0.5% of their calories as omega 3's, fish, to survive in cold-water environments, require at least 1%, twice the amount that humans require. This differential makes fish, fish oil, and algae (phytoplankton constituting the largest mass of greenery on the planet) valuable sources of omega-3 fatty acids.

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Summary

Providing patients with historical information about the effect of technological changes on the macronutrients of processed food items may increase acceptance of basic nutritional recommendations. Adopting the recommendations listed here will improve nutritional value by replacing significant amounts of processed items with whole carbohydrates, beneficial fats, and high-quality proteins.

  1. Replace large amounts of stripped carbohydrate (white flour, white rice, and sugar) with intact carbohydrates such as vegetables, beans, fruits, and whole grains.
  2. Restore beneficial ratios of omega-6 to omega-3 fatty acids by 1) increasing consumption of fish and produce, especially leafy, green vegetables, and 2) decreasing intake of processed food items.
  3. Avoid products that include partially hydrogenated fats in their ingredient list, even if the packaging describes the product as "trans-fat free".
  4. Increase intake of nutritious fats with nuts, avocadoes, seeds, fish, and even dark green leafy vegetables.
  5. Consume animal-based protein (fish, eggs, poultry) from entities that feed livestock a diet with a balanced ratio of omega-3 to omega-6 fatty acids.

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Suggested Readings

  • Alberti KG, Eckel RH, Grundy SM, et al. Harmonizing the metabolic syndrome: a joint interim statement of the International Diabetes Federation Task Force on Epidemiology and Prevention; National Heart, Lung, and Blood Institute; American Heart Association; World Heart Federation; International Atherosclerosis Society; and International Association for the Study of Obesity [published online ahead of print October 5, 2009]. Circulation 2009; 120:1640–1645. doi:10.1161/CIRCULATIONAHA.109.192644
  • Allport S. The Queen of Fats: Why Omega-3s Were Removed From The Western Diet and What We Can Do To Replace Them. Berkeley, CA: University of California Press; 2006.
  • Barlow SE. Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity: summary report. Pediatrics 2007; 120(suppl 4):S164–S192.
  • Diner HR. Hungering for America. Cambridge, MA: Harvard University Press; 2001.
  • Kannel WB, McGee DL. Diabetes and cardiovascular disease: The Framingham study. JAMA 1979; 241:2035–2038.
  • Karnik S, Kanekar A. Childhood obesity: a global public health crisis. Int J Prev Med 2012; 3:1–7.
  • Pearson TA, Blair SN, Daniels SR, et al. AHA guidelines for primary prevention of cardiovascular disease and stroke: 2002 update: consensus panel guide to comprehensive risk reduction for adult patients without coronary or other atherosclerotic vascular diseases. Circulation 2002; 106:388–391.
  • Pearson TA, Mensah GA, Alexander RW, et al. Markers of inflammation and cardiovascular disease: application to clinical and public health practice: a statement for healthcare professionals from the Centers for Disease Control and Prevention and the American Heart Association. Circulation 2003; 107:499–511.
  • Pollan M. The Omnivore's Dilemma: A Natural History of Four Meals. London: Penguin Press; 2006.
  • Reed W. The History of Sugar and Sugar Yielding Plants. London: Longmans, Green & Co.; 1866.
  • Simopoulos AP. The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases [published online ahead of print April 11, 2008]. Exp Biol Med (Maywood) 2008; 233:674–688. doi:10.3181/0711-MR-311
  • Tuomilehto J, Lindstrõm J, Eriksson JG, et al; for the Finnish Diabetes Prevention Study Group. Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. N Engl J Med 2001; 344:1343–1350.

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